1. Field of the Invention
The present invention relates to novel lacquer polyisocyanates with blocked isocyanate groups, which are characterized by the presence of amide and/or imide structural units, to a process for the production of these lacquer polyisocyanates and to the use thereof in combination with organic hydroxyl compounds as a stoving lacquer or for the production of a stoving lacquer.
2. Description of the Prior Art
It has been known for many years to produce lacquer coated wires using polyurethane-based wire coating compositions, such that the wires could be soldered at comparatively low temperatures of approximately 370.degree. to 390.degree. C. The binders of these wire coating compositions were based upon combinations of polyester polyols and phenol- or alkanol-blocked polyisocyanates (DE-AS 1,170,096 or DE-AS 2,626,175). Solderability may be improved by increasing the urethane group content in the polymer film by combining blocked polyisocyanates with hydroxy-functional oligourethanes (DE-OS 1,644,794).
These polyurethane coating compositions for wire are widely used for the production of easily solderable, lacquer coated copper wires, but they are not usable in all applications because they have only moderate thermal stability (temperature index TI according to IEC 172 of 130 to 155.degree. C., IEC=International Electrotechnical Commission). The temperature classes or indices of the insulating materials for wire lacquers are stated, for example, in S. Darling, International Wire Standards--Progress Towards Harmonisation in Proceedings 19th EEI Conference, Chicago, 25-28 September 1989, page 56.
It is also known to provide metal wires with coatings having much better long-term thermal stability (temperature index TI according to IEC 172 of approximately 180.degree. C.). The coating compositions used for the production of these lacquer coatings are based not on polyurethanes, but instead on heat resistant plastics, such as polyhydantoins (for example FR-PS 1,484,694, DE-PS 2,460,206), polyamideimides (for example DE-OS 3,544,548, DE-OS 3,714,033, DE-OS 3,817,614) or polyester-amideimides (for example U.S. Pat. No. 3,652,471, U.S. Pat. No. 4,997,891, DE-PS 3,249,497). However, none of the wires coated with wire coating compositions based on these plastics is solderable at below 400.degree. C. While the S. Darling publication does describe polyesterimides having a temperature index TI of 180.degree. C. as "solderable" (brazeable, solderable), such solderability according to IEC 851 is only achieved at temperatures of above 400.degree. C. The requirements for various types of lacquer coated wires are described in IEC 317. In this document, a soldering temperature of 470.degree. C. is, for example, set forth for solderable polyesterimides having a temperature index of 180.degree. C. (IEC 317, part 23).
Polyesterimides having an increased hydroxyl group content in combination with heat resistant blocked polyisocyanates are solderable at 370.degree. C., but, in comparison with conventional polyesterimides, exhibit sharply decreasing properties with regard to the tan .delta. critical point and softening temperature. The amideimide/polyurethane combinations described in EP 365,877 also exhibit similar disadvantages.
Polyisocyanates containing carbodiimide and/or uretoneimine groups and their use for in wire coating compositions are disclosed in EP-B 231,509. Depending upon the reaction partner, these polyisocyanates are suitable for the production of solderable lacquer coated wires (example 1 in EP-B 231,509) or heat-resistant lacquer coated wires (example 3 in EP-B 231,509).
Lacquer binders constituting mixtures of very specific polyamideimides based on trimellitic anhydride and the specific blocked polyisocyanates according to EP-B 231,509 are also known and, according to EP-A 291,699, yield highly heat resistant films.
It is also already known to use low molecular weight additives in binder systems based on blocked isocyanates in order to improve technical properties. According to EP-A 287,947, heat resistant lacquer coated wires are obtained by using unsaturated carboxylic acids in combination with polyisocyanates containing carbodiimide and/or uretoneimine groups. Using N,N',N"-tris-(2-hydroxyethyl)-isocyanurate as an additive for polyurethane-based wire lacquers also provides heat resistant lacquer coated wires (DE-OS 3,133,571).
In summary, it is apparent that the prior art is directed to obtaining lacquer coatings with elevated heat resistance (temperature index TI approximately 180.degree. C.) or lacquer coated wires which are solderable at a comparatively low temperature (approximately 370.degree. C.).
An object of the present invention is to provide a coating composition for heat resistant substrates, in particular for wire coating, which combines both of these advantages, i.e., elevated heat resistance and solderability at comparatively low temperatures. Wires, which are coated with coating compositions according to the invention, should in particular be solderable at approximately 370.degree. C. according to IEC 851 and should have a temperature index TI according to IEC 172 of at least 180.degree. C. In addition, the remaining properties should match the properties of known wire coating compositions or the resulting coated wires to the extent possible.
This object may be achieved with the lacquer polyisocyanates described below in greater detail or with their use in combination with organic polyhydroxyl compounds in stoving lacquers, in particular for wire coatings.